1. Field of the Invention
This invention relates to a synthesis of some components of the Boll Weevil sex attractant. More particularly this invention relates to a novel synthesis of the compounds 1a and b and 2a and b (numbers refer to the formulae shown on the accompanying drawings).
2. Brief Description of the Prior Art
The compounds 1a and b and 2a are known to be components of the sex pheromone of the male Boll Weevil (Anthonomus Grandis). The sex pheromone can play a prominent part in selective and relatively low toxic ways of fighting this noxious insect. It is therefore important that the components of the sex pheromone are easily accessible. The compounds 1a and b and 2a have been prepared in several ways. However either difficultly accessible and expensive starting materials had to be used, or many and/or difficult reaction steps had to be carried out. The majority of the known synthesis routes starts with 3,3-dimethylcyclohexanone. The first reaction step may be:
A. A Reformatsky-type reaction with zinc and ethyl bromo-acetate. see: J. H. Tumlinson et al., J. Org. Chem. 36 (1971), p. 2617 and U.S. Pat. Nos. 3,813,443 and 3,895,078. PA1 B. A Wittig-type reaction. see: O. P. Vig et al., J. Indian Chem. Soc. 49 (1972), p. 1181. PA1 C. A Wittig-type reaction (Horner modification), see: J. H. Babler and T. R. Mortell, Tetrahedron Letters 1972, p. 669. PA1 D. A condensation reaction with acetylene, see: C. A. Vodoz and H. Schinz, Helv. Chim. Acta 33 (1950), p. 1321.
In British Pat. No. 1,341,015 a synthesis route is described wherein addition of HCl to myrcene at -50.degree. C. is used as the first reaction step.
Isophorone is a cheap and abundantly available material. Formylation of isophorone (compound 3) on its single methyl group would provide a compound with the carbon skeleton of aldehyde 1 (a and/or b) although in a different oxidation state. However, formylation of unsaturated ketones is limited to a very few examples and is known to produce almost exclusively polyformylated products. (see e.g. V. M. Vlasov and O. V. Zakharova, Zh. Org. Khim. 10 (1974), p. 66 cf. Chem. Abstracts 80 (1974), 108113e; Z. Arnold and A. Holy, Coll. Czech. Chem. Comm. 30 (1965), p. 47; A. Holy and Z. Arnold, Coll. Czech. Chem. Comm. 30 (1965), p. 53.)
It was now found that compounds 1a and b may be easily obtained from isophorone applying as the essential reaction steps a formylation reaction, preferably a Vilsmeier formylation reaction, and a catalytic hydrogenation, as outlined in reaction schemes A and B, shown in the accompanying drawings.
Vilsmeier reactions are described, for example, by G. A. Olah and S. J. Kuhn in "Friedel-Crafts and Related Reactions" edited by G. A. Olah, 3, Part 2, p. 1211, Interscience Publishers (1964), G. Hazebroucq, Annales Pharmaceutiques Francaises, 24, (1966) p. 793; V. I. Minkin and G. N. Dorofeenko, Russian Chem. Review (Uspekhi Khimi) 29 (1960) p. 599.